Materials Science Forum Vols. 638-642

Abstract: TiC−Fe system cermets and TiC−SUS310L (TiC−SS) system cermets with fully relative density have been produced from elemental powders via self-propagating high-temperature synthesis (SHS) reaction combined with pseudo-hot isostatic pressing. X-ray diffraction (XRD) analysis showed that the SHS products consist of TiC and the binder phase of Fe or SS. Metallographic analysis revealed TiC particles of several micrometer size were homogeneously dispersed in the binder phase. Regardless of differences in the binders, both cermets have similar tendencies: their relative densities increased and their hardness and compressive strengths decreased with increases in the volume fraction of either Fe or SS. In addition, the cermets used stainless steel as the binder had twice superior corrosion resistance to those used iron and also their hardness and compressive strength showed excellent values.

Abstract: The lotus-type porous carbon steel with cylindrical pores has been fabricated by continuous zone melting technique in pressurized mixture of hydrogen and helium gases. In order to investigate the mechanical properties, the tensile and compression tests were carried out. The ultimate tensile strength of the specimen with cylindrical pores parallel to the solidification direction is lower than the estimated value assuming that the strength is decreased in proportion to decreasing cross section area of the specimen, while the yield strength is higher than that estimated. The compressive yield strength is also higher than that estimated. The increase in yield strength is attributed to the precipitation strengthening. The tensile strength is increased by quenching and tempering, while the elongation decreases. Such mechanical properties are discussed in terms of microstructureal analysis. Furthermore, simulations of the mechanical properties by FEM analysis were carried out.

Abstract: Since the introduction of CoCrMo alloy metal-on-metal hip replacements have shown a great clinical performance. Metal-on-metal couplings produce a much lower wear rate and volume than e.g. metal-on-polyethylene. However, the particle size is significantly smaller within a nm-range. To evaluate the formation of nano-size wear particles in metal-on-metal hip replacements it is essential to understand the micro-structural changes in the sub-surface region of the CoCrMo alloy. For this study a MoM hip implant was analyzed by means of TEM. The results revealed that the good wear performance of this CoCrMo alloy is linked to a strain induced fcc  hcp phase transformation and in-situ re-crystallization under high shear stresses. The result is a nano-crystalline surface zone of ~200 to 400 nm thickness which undergoes an ongoing process of mechanical intermixing with componants of the interfacial fluid. The incorporation of organic carbon from proteins in between the nano-crystals could be visualised by EFTEM and EDS. This mechanically mixed nc-zone must be the origin of the wear particle detachment. An earlier study by Catelas et. al confirms the hypothesis of the location of wear particle detachment by analyzing the shape and chemical composition of emitted wear particles which exhibits the same size and shape of crystals observed in the nc-zone of the implant analyzed in this study.

Abstract: The phenomenological constitutive framework for compressible elasto-plastic solids presented by Chen and Lu [1] is extended to the dynamic cases by assuming that the material parameter curves in the stress potential depend also on the strain rate. To check the applicability of the extended model, three types of dynamic experiments, i.e., uniaxial compression, lateral-constrained compression and side-constrained compression tests, are conducted for an open-cell aluminum foam at different strain rates. The first two types of dynamic tests are used as characteristic tests to determine the material parameter curves at different strain rates which are then used to construct the stress potential function in the model. The results show that the stress-strain curves under side-constrained compression predicted by the model are in agreement with those obtained experimentally.

Abstract: Sintered metal fiber structures show a favourable ratio between pressure drop and inner surface area. Their exclusively open-cell morphology makes them well suited for heat transfer or temporary heat storage applications. Recently, highly conductive sintered metal fiber structures were successfully prepared from melt extracted aluminum alloy fibers. The heat conduction and fluid flow properties of metallic sintered short fiber structures were determined experimentally and compared with simple analytical models. It was found that equations taken from the available literature yield good approximations to the experimental results.

Abstract: Porous aluminum alloy has been developed by powder metallurgy route using Spark plasma sintering (SPS) technique. Sintered material was produced by SPS system after getting the mixture of Al-12Si alloy and titanium hydride powders. Porous materials are prepared under various process conditions, and the pore morphology was investigated. Compression test is performed at crosshead speed of 1mm/min, 10mm/min and 100mm/min with no lubricant. The compression strength, σC i.e. plateau stress was estimated 12MPa at the density of porous materials, 0.7 Mg/m3. Densification strain εD from compression curve is around 0.6. These properties depend on pore morphology of porous materials, and it is possible to control the morphology under specific condition with this process. Plateau stress and absorbed energy of heat treated porous Al-Si alloy were estimated by measurement of a first peak stress and calculated an area up to 0.5 strain from compressive stress-starin curves. Young’s modulus is measured by starin gauge method under compression test. Porous aluminum alloy filled mold die is also produced successfully.

Abstract: In the present study, ultra fine-grained low carbon steel samples were processed by equal channel angular pressing (ECAP). Mechanical properties of the specimens annealed statically at several temperatures were evaluated by tensile and hardness test. In addition, grain sizes of the specimens were measured by SEM-electron back scattering pattern (SEM-EBSP) and X-ray diffraction analysis. Differential scanning calorimetry (DSC) measurement also evaluated thermal reactions in anneal process of the specimen. As a result, the grain size was changed at the temperature between 550oC and 600oC drastically and the tensile strength also became lower at the same temperature. The relation between yield stress and averaged grain diameter of specimens obeyed the Hall-Petch relation except the normalized specimen. Behavior of grain growth and recovery in structural observation by EBSP corresponded to reaction signal of the DSC curve.

Abstract: The structural recrystallization mechanisms operating in an Fe – 27%Cr – 9% Ni dual-phase (ferrite-austenite) stainless steel after large strain processing to total strain of 4.4 were investigated in the temperature range of 400-700oC. The severe deformation resulted in the development of an ultrafine grained microstructure consisting of highly elongated grains/subgrains with transverse dimensions of 160 nm and 130 nm in ferrite and austenite, respectively. The annealing mechanism operating in ferrite phase was considered as continuous recrystallization, which involved recovery leading to the development of essentially polygonized microstructure. On the other hand, the mechanism of discontinuous nucleation took place at an early recrystallization stage in austenite phase.

Abstract: There is increasing interest in using Al alloy sheets for auto body applications. However Al alloys exhibit poor drawability as indicated by low values of the normal anisotropy, rm. Techniques for improving the value of rm rely on developing a favourable shear texture in the sheet. In this study, Al alloy AA 6061 sheets of dimensions 225 mm x 200 mm and 1 mm thick were subjected to severe plastic deformation by repeated groove pressing using a set of grooved and flat dies alternatively. The orientation of the grooves with respect to the rolling direction was also varied. Microstructure characterization and mechanical property measurements were carried out. X- ray diffraction scans were carried out to measure the relative intensities of the (111) and (200) peaks. The r values was measured as per ASTM standard E 517 on strip specimens cut at 0°, 45° and 90° to the rolling direction and the normal anisotropy value (rm) and planar anisotropy value (Δr) values were determined. The limiting drawing ratio (LDR) was determined using the Swift cupping test techniques. It was observed that the rm values increased from 0.72 in the as received condition to a maximum of 0.94 and the LDR increased from 1.93 to 2.06 when the groove pressing was carried out with grooves at to 45° the rolling direction. The improvement in rm values can be correlated to the texture developing in the sheet as a result of severe plastic deformation.

Abstract: Equal Channel Angular Rolling (ECAR), the severe plastic deformation process, is suitable for shear deforming long and thin sheet continuously. An interesting issue is that thickness of a sheet is not reduced during ECAR. Although shear texture and fine grain structure in Al alloys are easily obtained by ECAR, yet the ECAR process’s difficulties in terms of technical control still remain, such as surface defect, low ductility and low processing speed. The surface defects and processing speed are partially improved by applying a series deformation of rolling and ECAR. A high-speed solution heat-treatment is developed for restoring the ductility of Al 6061 alloy.